The present invention relates to a hydrocarbon conversion process. More particularly, this invention relates to the catalytic hydrocracking of hydrocarbons.
The hydrocracking of hydrocarbons is old and wellknown in the prior art. These hydrocracking processes can be used to hydrocrack various hydrocarbon fractions such as reduced crudes, gas oils, heavy gas oils, topped crudes, shale oil, coal extract and tar extract wherein these fractions may or may not contain nitrogen compounds. Modern hydrocracking processes were developed primarily to process feeds having a high content of polycyclic aromatic compounds, which are relatively unreactive in catalytic cracking. The hydrocracking process is used to produce desirable products such as turbine fuel, diesel fuel, and middle distillate products such as naphtha and gasoline.
The hydrocracking process is generally carried out in any suitable reaction vessel under elevated temperatures and pressures in the presence of hydrogen and a hydrocracking catalyst so as to yield a product containing the desired distribution of hydrocarbon products.
Hydrocracking catalysts generally comprise a hydrogenation component on an acidic cracking support More specifically, hydrocracking catalysts comprise a hydrogenation component selected from the group consisting of Group VIA metals and Group VIII metals of the Periodic Table of Elements, their oxides or sulfides. The prior art has also taught that these hydrocracking catalysts contain an acidic support comprising a crystalline alumino silicate material such as X-type and Y-type alumino silicate materials. This crystalline aluminosilicate material is generally suspended in a refractory inorganic oxide such as silica, alumina, or silica-alumina.
The preferred Group VIA metals are tungsten and molybdenum the preferred Group VIII metals are nickel and cobalt.
The prior art has also taught that combinations of metals for the hydrogenation component, expressed as oxides and in the order of preference, are: NiO.sub.3, NiO-MoO.sub.3, CoO-MoO.sub.3, and CoO-WO.sub.3.
Other hydrogenation components broadly taught by the prior art include iron, ruthenium, rhodium, palladium, osmium, indium, platinum, chromium, molybdenum, vanadium, niobium, and tantalum.
References that disclose hydrocracking catalysts utilizing nickel and tungsten as hydrogenation components, teach enhanced hydrocracking activity when the matrix or catalyst support contains silica-alumina. For instance, U.S. Pat. Nos. 4,576,711, 4,563,434, and 4,517,073 all to Ward et al., show at Table V thereof that the lowest hydrocracking activity is achieved when alumina is used in the support instead of a dispersion of silica-alumina in alumina. The lowest hydrocracking activity is indicated by the highest reactor temperature required to achieve 60 vol. % conversion of the hydrocarbon components boiling above a predetermined boiling range temperature end point to below that boiling range temperature end point.
Similarly, U.S. Pat. No. 3,536,605 to Kittrell et al. teaches the use of silica-alumina in the catalyst support when a nickel- and tungsten-containing hydrogenation component is employed.
U.S. Pat. No. 3,598,719 to White teaches a hydrocracking catalyst that can contain no silica; however, the patent does not present an example showing the preparation of a catalyst devoid of silica nor does the patent teach the preferential use of nickel and tungsten as hydrogenation metals.
As can be appreciated from the above, there is a myriad of catalysts known for hydrocracking whose catalytic properties vary widely. A catalyst suitable for maximizing naphtha yield may not be suitable for maximizing the yield of turbine fuel. Further, the degree of cracking and yield structure is also dependent upon the feedstock composition.
Catalysts of high hydrogenation activity relative to acidity yield more highly saturated products as required in distillate fuels such as jet or aviation fuel.
Reconciling hydrodenitrogenation activity with hydrocracking activity in a single hydrocracking catalyst presents a difficulty. For instance when a feedstock having a high nitrogen content is exposed to a hydrocracking catalyst containing a high amount of cracking component the nitrogen serves to poison or deactivate the cracking component. Another difficulty is presented when the hydrocracking process is used to maximize naphtha yields from a feedstock containing light catalytic cycle oil which has a very high aromatics content. The saturation properties of the catalyst must be carefully gauged to saturate only one aromatic ring of a polynuclear aromatic compound such as naphthalene in order to preserve desirable high octane value aromatic-containing hydrocarbons for the naphtha fraction. If the saturation activity is too high, all of the aromatic rings will be saturated and subsequently cracked to lower octane value paraffins.
On the other hand, distillate fuels such as diesel fuel or aviation fuel have specifications that stipulate a low aromatics content. This is due to the undesirable smoke production caused by the combustion of aromatics in diesel engines and jet engines.
Prior art processes designed to convert high nitrogen content feedstocks and produce jet fuel are usually two stage processes wherein the first stage is designed to convert organic nitrogen compounds to ammonia prior to contacting with a hydrocracking catalyst which contained a high amount of cracking component; e.g., a molecular sieve material.
For instance U.S. Pat. No. 3,923,638 to Bertolacini et al. discloses a two catalyst process suitable for converting a hydrocarbon containing substantial amounts of nitrogen to saturated products adequate for use as jet fuel. Specifically, the subject patent discloses a process wherein the hydrodenitrogenation catalyst comprises as a hydrogenation component a Group VIA metal and group VIII metal and/or their compounds and a cocatalytic acidic support comprising a large-pore crystalline aluminosilicate material and refractory inorganic oxide. The hydrocracking catalyst comprises as a hydrogenation component a Group VIA metal and a Group VIII metal and/or their compounds, and an acidic support of large-pore crystalline aluminosilicate material. For both hydrodenitrogenation catalyst and the hydrocracking catalyst, the preferred hydrogenation component comprises nickel and tungsten and/or their compounds and the preferred large-pore crystalline aluminosilicate material is ultrastable Y, large-pore crystalline aluminosilicate material.
A two-stage process suitable for maximizing gasoline-boiling range products is disclosed in U.S. Pat. No. 3,649,523 to Bertolacini et al. The disclosed first stage is a hydrotreating stage wherein nitrogen and sulfur are removed from the feedstock followed by a hydrocracking stage employing a catalyst containing a Group VIA metal, a Group VIII metal, a large-pore crystalline aluminosilicate material, and a porous support selected from the group consisting of alumina, aluminum-phosphate, and silica. The patent exemplifies several cobalt molybdenum-containing catalysts with the one containing a silica-alumina matrix material possessing the highest activity.
It has also been discovered that denitrogenation, hydrocracking, and polyaromatic saturation activities can be maximized with a single catalyst system when treating a feedstock containing highly aromatic light catalytic cycle oil. Specifically, application Nov. 124,280 filed on Nov. 23, 1987, the teachings of which are incorporated herein by reference, discloses a catalyst comprising a combination of a nickel component, and a tungsten component coupled with a support component comprising a crystalline molecular sieve material component, and an alumina component. This catalyst system has been discovered to provide increased selectivity towards high octane naphtha with decreased undesirable selectivity towards C.sub.1 to C.sub.5 light gas.
It has now been discovered that the activity of the above-described hydrocracking process can be markedly increased by hydrotreating the feedstock prior to passing it to the hydrocracking process.
An attendant advantage of increasing the activity of the catalyst in the hydrocracking stage is the ability to increase the throughput of feed to the hydrocracking process unit.
The process of the invention also surprisingly yields a naphtha fraction having a high octane affording aromatics content. The amount of desirable aromatics produced by the process of the invention is significantly higher than when the feedstock is hydrocracked without an initial hydrotreatment step.